Evolution of the branchiostegal membrane and restricted gill openings in Actinopterygian fishes

A phylogenetic survey is a powerful approach for investigating the evolutionary history of a morphological characteristic that has evolved numerous times without obvious functional implications. Restricted gill openings, an extreme modification of the branchiostegal membrane, are an example of such a characteristic. We examine the evolution of branchiostegal membrane morphology and highlight convergent evolution of restricted gill openings. We surveyed specimens from 433 families of actinopterygians for branchiostegal membrane morphology and measured head and body dimensions. We inferred a relaxed molecular clock phylogeny with branch length estimates based on nine nuclear genes sampled from 285 species that include all major lineages of Actinopterygii. We calculated marginal state reconstructions of four branchiostegal membrane conditions and found that restricted gill openings have evolved independently in at least 11 major actinopterygian clades, and the total number of independent origins of the trait is likely much higher. A principal component analysis revealed that fishes with restricted gill openings occupy a larger morphospace, as defined by our linear measurements, than do fishes with nonrestricted openings. We used a decision tree analysis of ecological data to determine if restricted gill openings are linked to certain environments. We found that fishes with restricted gill openings repeatedly occur under a variety of ecological conditions, although they are rare in open‐ocean pelagic environments. We also tested seven ratios for their utility in distinguishing between fishes with and without restricted gill openings, and we propose a simple metric for quantifying restricted gill openings (RGO), defined as a ratio of the distance from the ventral midline to the gill opening relative to half the circumference of the head. Functional explanations for this specialized morphology likely differ within each clade, but its repeated evolution indicates a need for a better understanding of diversity of ventilatory morphology among fishes. J. Morphol. 276:681–694, 2015. © 2015 Wiley Periodicals, Inc.     

Evidence for Permo-Triassic colonization of the deep sea by isopods

The deep sea is one of the largest ecosystems on Earth and is home to a highly diverse fauna, with polychaetes, molluscs and peracarid crustaceans as dominant groups. A number of studies have proposed that this fauna did not survive the anoxic events that occurred during the Mesozoic Era. Accordingly, the modern fauna is thought to be relatively young, perhaps having colonized the deep sea after the Eocene/Oligocene boundary. To test this hypothesis, we performed phylogenetic analyses of nuclear ribosomal 18S and 28S and mitochondrial cytochrome oxidase I and 16S sequences from isopod crustaceans. Using a molecular clock calibrated with multiple isopod fossils, we estimated the timing of deep-sea colonization events by isopods. Our results show that some groups have an ancient origin in the deep sea, with the earliest estimated dates spanning 232–314 Myr ago. Therefore, anoxic events at the Permian–Triassic boundary and during the Mesozoic did not cause the extinction of all the deep-sea fauna; some species may have gone extinct while others survived and proliferated. The monophyly of the ‘munnopsid radiation’ within the isopods suggests that the ancestors of this group evolved in the deep sea and did not move to shallow-water refugia during anoxic events.     

Proliferation of elongate fishes in the deep sea

It was hypothesized that energetically efficient anguilliform swimming and axial elongation in fishes is favoured in the deep sea and predicted that the degree of elongation of the body form of fishes would increase with depth. An index of fish shape was derived from the relationship between length and mass. This was combined with data on abundance of c. 266 fish species from 389 research trawl tows made at depths of between 300 and 2030 m in the north‐east Atlantic Ocean. The degree of elongation of the fish increased with depth to c. 1250 m before levelling off. The strength of this phenomenon varied between higher level taxa, being most apparent in the Gadiformes and Osmeriformes, and weak or absent in the Perciformes and Selachimorpha. The advantage of efficient elongate body forms may explain why certain taxa such as the grenadiers (Macrouridae) have dominated the deep sea, some have restricted depth ranges, e.g. the sharks, skates and rays, and others are almost entirely absent, e.g. the flatfishes (Pleuronectiformes).     

The early evolution of ray‐finned fishes

Ray‐finned fishes (Actinopterygii) constitute approximately half of all living vertebrate species. A stable hypothesis of relationships among major modern lineages has emerged over the past decade, supported by both anatomy and molecules. Diversity is unevenly partitioned across the actinopterygian tree, with most species concentrated within a handful of geologically young (i.e. Cretaceous) teleost clades. Extant non‐teleost groups are portrayed as ‘living fossils’, but this moniker should not be taken as evidence of especially primitive structure: each of these lineages is characterized by profound specializations. Attribution of fossils to the crowns and apical stems of Cladistia, Chondrostei and Neopterygii is uncontroversial, but placements of Palaeozoic taxa along deeper branches of actinopterygian phylogeny are less secure. Despite these limitations, some major outlines of actinopterygian diversification seem reasonably clear from the fossil record: low richness and disparity in the Devonian; elevated morphological variety, linked to increases in taxonomic dominance, in the early Carboniferous; and further gains in taxonomic dominance in the Early Triassic associated with earliest appearance of trophically diverse crown neopterygians.     

A phylogenetic analysis of the 18S ribosomal RNA sequence of the coelacanth Latimeria chalumnae

Synopsis Approximately 98% of the sequence of the 18S ribosomal RNA (rRNA) of the coelacanth Latimeria chalumnae was determined by a combination of direct RNA sequencing and sequencing of rRNA genes amplified by the polymerase chain reaction. This sequence was compared with 18S rRNA sequences of similar length from seven other vertebrate species, representing the taxa Petromyzontiformes, Holocephali, Elasmobranchii, Actinopterygii, Dipnoi, Amphibia, and Amniota, in order to determine the most likely sister group of the coelacanth. Maximum parsimony analysis of these sequences resulted in a single most parsimonious tree containing a number of anomalous relationships among these groups. A bootstrap analysis showed that none of the relationships in this tree was significantly supported at the 95% level, however. Addition of data from 15 other vertebrates (providing multiple representatives of most of the higher taxa) resulted in similar ambiguous groupings, as did a number of methods of editing the sites compared (designed to eliminate rapidly evolving positions). These results may be due to a relatively rapid radiation of the major lineages of osteichthyans, the resolution of which will require molecular information from a larger portion of the coelacanth genome.     

The histology and affinities of sinacanthid fishes: primitive gnathostomes from the Silurian of China

On the basis of well preserved specimens from the Lower Silurian of the Tarim Basin, Xinjiang Uygur Autonomous Region and Shiqian County, Guizhou Province, People's Republic of China we describe in detail the histological structure of sinacanthid spines, the only known remains of a group of fishes common in Silurian strata from China. The sinacanthids have previously been assigned either to the acanthodians or to the chondrichthyans. The spine structure is composed of an outer layer of atubular dentine and an inner layer of globular calcified cartilage, and the nature and distribution of these tissues indicates that the spines were formed as a result of interaction between the endoskeleton and dermoskeleton. The tissue distribution and style of growth described herein places the sinacanthids crownwards of the placoderms, and possibly within the total group Chondrichthyes. However, before they can be firmly placed within a phylogenetic scheme, further evidence is required both on the general anatomy of sinacanthids and on the nature of chondrichthyan apomorphies.  © 2005 The Linnean Society of London, Zoological Journal of the Linnean Society , 2005, 144 , 379–386.     

Patterns of genome size diversity in the ray-finned fishes

The ray-finned fishes make up about half of all vertebrate diversity and are by far the best represented group in the Animal Genome Size Database. However, they have traditionally been the least well investigated among vertebrates in terms of patterns and consequences of genome size diversity. This article synthesizes and expands upon existing information about genome size diversity in ray-finned fishes. Specifically, compiled data from the Animal Genome Size Database and FishBase are used to examine the potential patterns of interspecific genome size variability according to ecology, environment, morphology, growth, physiology, reproduction, longevity, and taxonomic diversity. Polyploidy and haploid genome sizes are considered separately, revealing differences in their respective consequences. This represents the most comprehensive summary of fish genome size diversity presented to date, and highlights areas of particular interest to investigate as more data become available.

Taxonomy of monogeneans of deep sea fishes in southeastern Australia

The following monogeneans from deepwater fish in southeastern Australia are described, based on a survey of 1,563 fish (66 or 67 species, 35 families, 15 orders): Reimericotyle ceratoscopeli from Myctophum phengodes, M. hygomi, Hygophum hygomi and Ceratoscopelus warmingii; Diclidophora tubiformis n. sp. from Coryphaenoides serrulatus and C. subserrulatus; Diclidophora sp. from Lepidorhynchus denticulatus; Polycliphora nezumiae from Coryphaenoides serrulatus; Paracyclocotyla sp. from Lepidion microcephalus; juvenile Paracyclocotyla sp. from Hoplostethus atlanticus; Polyipnicola hygophi from Hygophum hygomi, Notoscopelus resplendens, Electrona risso and Myctophum phengodes; Diclidophoropsis sp. from Nezumia sp.; Eurysorchis manteri from Hyperoglyphe sp.; Heteraxinoides sp. from Synagrops japonicus; Megalocotyle helicoleni from Helicolenis papillosus. The following species are recorded but not described: Allocotylophora polyprionum (Diclidophoroidea), four unidentified species of the Diclidophoroidea from Diastobranchus capensis, both Hoplostethus atlanticus and H. intermedius, Chlorophthalmus sp. and Synagrops japonicus, respectively, one species of the Capsaloidea from Enoplosus armatus, one species of the Capsalidae from Lepidotrigla argus, one species of the Dactylogyrinae from Atypichthys strigatus, and one species of the Ancyrocephalinae from Chlorophthalmus nigripinnis.     

Some peculiarities of brain phospholipids in deep sea fishes]

Total phospholipids (PL) as well as the content of various phospholipid classes and their fatty acid composition have been investigated in the brain of mesopelagic and abyssal marine teleosts. These species were compared to shallow water ones. The brain of deep sea fishes was found to be very poor in PL as compared to the brain of mesopelagic ans surface water species. No differences concerning the brain PL content were revealed between the two last mentioned groups. The relative content of separate PL classes was very similar in all the species studied irrespectively of the depth of their habitat. Peculiarities were found in fatty acid composition of individual PL from deep sea species as compared to surface ones. The deeper the habitat, the lower the content of saturated fatty acids, especially of the stearic acid. The lowest content of saturated fatty acids, maximum level of polyenoic fatty acids as well as some peculiarities in the relative content of particular fatty acids were found in the brain of ultraabyssal (6, 000 m) Leucicorus sp.     

Colonization of an artificial stream by fishes and aquatic macro-invertebrates

We examined colonization by fishes and macro-invertebrates from permanent streams into an artificial freshwater stream simulating lotic temporary bodies of water that exist for only a limited period each year. After introducing water, invertebrates such as chironomid larvae in mud increased in numbers rapidly in the experimental stream, although they were rare in mud in the permanent streams. Eleven of 12 fish species present in the permanent streams colonized the experimental stream and preyed upon invertebrates, although fish composition differed significantly between the two streams. About 100 days after the initiation of the experiment, both species richness and diversity in the experimental stream reached almost the same level as that in the permanent streams. More diverse fishes colonized the complex section where habitat diversity was high compared to the simple section in the experimental stream. Our study strongly suggests that lotic temporary waters such as temporary streams around main rivers have unique ecological characteristics and serve as valuable foraging sites for fish.     

Colonization of the pelagic realm by calanoid copepods

The evolution of calanoid copepods probably extends back into the mid-Paleozoic. Environmental change from the Paleozoic through to the Tertiary is reviewed. Turbidity, water clarity, oxygen, pelagic primary production, and tectonically induced changes in the morphology of the oceans are probably all important drivers of calanoid evolution and their invasion of the pelagic realm. Current views of the phylogeny of the Calanoida are presented as well as a review of some recent work on metabolic potential, female genital system, and nervous system. It is hypothesized that ancestors of the Arietelloidea and Diaptomoidea invaded the water column in the Devonian at a similar time to the Ostracoda and that the ancestors of the Calanoidea–Clausocalanoidea, with their myelinated axons, arose in the Permian during the major deep ventilation of the ocean. Present day distributions of some Diaptomidae, Centropagidae, and Calanidae suggest that these families successfully came through the Jurassic/Cretaceous expansion of the oxygen minimum zone and the K-T boundary event. Some Arietelloidea and Clausocalanoidea became secondarily benthic and may have survived the K-T boundary event in this environment. It is postulated that some Clausocalanoidea reinvaded the water column (e.g. Clausocalanidae, Aetideidae, Phaennidae, Scolecitrichidae) in the Tertiary after finding refuge in deep, low oxygen water away from the sea surface. It is possible that these hypotheses may be testable using genetic information in the near future.     

Reproduction in Middle Triassic actinopterygians; complex fin structures and evidence of viviparity in fossil fishes

New well-preserved specimens of Peltopleurus lissocephalus Brough, 1939, a small actinopterygian fish of the Alpine Middle Triassic, are described. Some of them show a highly modified anal fin, with a brush-shaped holdfast device and a gonopodium-like structure, possibly used during mating behaviour. Additional examples of modified anal fins are reported for the genera Cephaloxenus and Habroichthys. Evidence for viviparity within the genus Saurichthys , found at the same localities, is discussed and compared with earlier reports of the phenomenon. A comparison is made of the different reproductive strategies used in some Middle Triassic actinopterygian fishes.     

Lamins of the sea lamprey (Petromyzon marinus) and the evolution of the vertebrate lamin protein family

Lamin proteins are found in all metazoans. Most non-vertebrate genomes including those of the closest relatives of vertebrates, the cephalochordates and tunicates, encode only a single lamin. In teleosts and tetrapods the number of lamin genes has quadrupled. They can be divided into four sub-types, lmnb1, lmnb2, LIII, and lmna, each characterized by particular features and functional differentiations. Little is known when during vertebrate evolution these features have emerged. Lampreys belong to the Agnatha, the sister group of the Gnathostomata. They split off first within the vertebrate lineage. Analysis of the sea lamprey (Petromyzon marinus) lamin complement presented here, identified three functional lamin genes, one encoding a lamin LIII, indicating that the characteristic gene structure of this subtype had been established prior to the agnathan/gnathostome split. Two other genes encode lamins for which orthology to gnathostome lamins cannot be designated. Search for lamin gene sequences in all vertebrate taxa for which sufficient sequence data are available reveals the evolutionary time frame in which specific features of the vertebrate lamins were established. Structural features characteristic for A-type lamins are not found in the lamprey genome. In contrast, lmna genes are present in all gnathostome lineages suggesting that this gene evolved with the emergence of the gnathostomes. The analysis of lamin gene neighborhoods reveals noticeable similarities between the different vertebrate lamin genes supporting the hypothesis that they emerged due to two rounds of whole genome duplication and makes clear that an orthologous relationship between a particular vertebrate paralog and lamins outside the vertebrate lineage cannot be established.